Electrical contact material

ABSTRACT

A reinforced metallic composite material suitable for use as an electrical contact material wherein refractory metal wires having a diameter of about 1 to about 50 mils are substantially surrounded by a soft metal material. The separation between adjacent wires may be little, if any, separation therebetween to a separation of up to about 40 mils therebetween. The reinforced metallic composite may be fabricated by infiltration of the soft metal into void spaces between the refractory wires in a hydrogen atmosphere or in a vacuum environment.

[ ELECTRICAL CONTACT MATERIAL [75] Inventors: Richard H. Krock, Peabody;

Edward J. Zdanuk, Lexington, both of Mass.

[73] Assignee: P. R. Mallory & Co. Inc.,

Indianapolis, Ind.

22 Filed: Apr. 22, 1971 21 Appl. N6; 136,543

Related US. Application Data [63] Continuation of Ser. No. 762,570, Sept. 25, 1968,

abandoned.

52 us c1... 29/630 C, 29/630 R, 200/264 5 1 Int. Cl HOlr 9/00 [58} Field of Search ..200/166 C, 166 F, 144, 2, 164/66, 25,25 G, 103, 332, 259, 29/527.6, 29/527.6, 419, 630 C, 76/105, 75/211,

[56] References Cited [111 3,852,879 [4 1 Dec. 10, 1974 1 11/1968 Krock et al. 29/630 C Primary EXaminer-C. W. Lanham Assistant Examiner-James R. Duzan Attorney, Agent, or Firm-Hoffman Meyer & Hanson 57 ABSTRACT 7 Claims, 5 Drawing Figures UNITED STATES PATENTS 3,254,189 5/1966 Evanicsko et al ..29/630C r I I PATENTEU [1E6 i 0 1974 INVENTORS RICHARD H. KROCK EDW D J. ZDAN K fig V\ C NEW n This application is a continuation of Ser. No. 762,570, filed Sept. 25, 1968, now abandoned.

The present invention relates to an electrical contact material and method for making the same and, more particularly, to an electrical contact material having an aligned refractory metal wire constituent and a soft metal constituent and methods for making the same.

Electrical contacts fabricated from a soft metal material such as copper, silver, gold and the like exhibit excellent electrical and thermal conductivity properties. However, when the soft metal contact material carries high current densities, the material, under these arcing conditions, tends to rapidly pit the working surface of the material, to erode the contact material at a high rate, to weld easily. the adjacent working surfaces of the contact material, and to form insulating surface reaction products when operated in chemically active environments.

Refractory metal contact materials fabricated from metals such as tungsten, molybdenum, rhenium, tantalum and the like exhibit resistance to are erosion, and superior anti-weld and hardness properties. However, the electrical resistivity of the refractory metal material is several times greater than that of the soft metal contact material which property significantly lowers the efficiency and reliability of the refractory metal contact material. The resistivity of the substantially pure refractory metal material is sufficiently high to limit its application as an electrical contact material.

Laminated materials suitable for use as electrical contact materials may be fabricated wherein a soft material is layered over a core of refractory metal material. Although an increase in the strength of the laminated material over the soft metal contact material is observed, erosion of the material may occur resulting in pitting of the contact materials which may cause discontinuous operation of the contact material under adverse environmental conditions. ln addition, the bond existing between the laminated layers is weak which may result in spalling or separation of the laminated layers at the interface between the layers. When spalling occurs, the erosion rate of the soft material increases therebyresulting in an electrical contact material having undesirable electrical properties.

1 The use of a refractory powder metal skelton such as tungsten or the like infiltrated with a soft metal having good electrical and thermal properties such as copper and the like provide a composite electrical contact material which attempts to take advantage of the properties of both of the constituents of the composite. The resultant contact material is a compromise between the strength of the refractory material and the electrical and thermal conductivity of the soft metal material. It should be noted thatthe strength of the composite is not as great as the pure refractory material and that the conductivity of the composite material is not as great as that of the pure copper material. However, the combination of these two properties of the composite material provides a desirable characteristic. Such a composite material has a working surface which is non-uniform in the percent by volume of soft metal present at different areas of the working surface and during operation the composition at the surfaces of contacts changes since loss of the soft metal component is greater than that of the refractory metal. The composites made from refractory metal powders also have a non-uniform particle to particle bonding which may lead to spalling of near surface layers during electrical switching operations. Each type of non-uniformity is detrimental to contact applications.

Therefore, it is an object of the present invention to provide a reinforced metallic composite electrical contact material which overcomes the problems enumerated hereinbefore.

A further object of the present invention is to provide a reinforced metallic composite material suitable for use as an electrical contact material having high strength and ductility and low electrical resistivity properties.

Yet another object of the present invention is to provide a reinforced metallic composite material suitable for use as an electrical contact material wherein the reinforcing material is substantially continuous wires of refractory metal substantially perpendicular to the working face of the contact having the voids therebetween substantially filled with a high conductivity soft metal.

Yet another object of the present invention is to provide a reinforced metallic composite material suitable for use as an electrical contact material wherein the strengthening metal materials are refractory wires in an hexagonal or other suitable array, the voids therebetween being substantially completely filled with a soft metal. 7

A further object of the present invention is to provide a reinforced metallic composite material wherein the reinforcing material is a refractory wire material having its longitudinal axis substantially perpendicularto the working face of the contact material and being separated from its adjacent refractory wire by no more than about three-fourths the diameter of the refractory wire.

Another object of the present invention is to provide a reinforced metallic composite material suitable for use as an electrical contact material wherein high strength metal wires are substantially surrounded by and dispersed in a lower strength, more ductile and more highly conductive metal materials. 7

Yet another object of the present invention is to. provide reinforced metallic composite material suitable for use as an electrical contactmaterial wherein the refractory metal wires having a diameter of about 1 to about 50 mils are substantially surrounded by a soft metal material wherein the separation between adjacent refractory wires is nearly zero to about 40 mils.

Yet another object of the present invention is to provide a reinforced metallic composite material suitable for use as an electrical contact material wherein the thickness of the contact material is reduced due to superior erosion resistance of the reinforced material.

A further object of the present inventionis to provide a reinforced metallic composite material suitable for use as an electrical contact material wherein the contact material has added strength in the desired direction so as to resist fractures parallel to the contact face and resist stresses generated by elevated operating temperatures experienced during contact operation.

'A further object of the present invention is to provide a method for inaking reinforced metallic composite materials suitable for use as electrical contact materials.

ture.

Yet still another object of. the present invention is to provide a reinforcedmetallic composite material suitable for use as an electrical contact material wherein a compromise between strength and electrical and thermal conductivity is achieved.

A further object of the present invention is to provide a reinforced metallic-composite material suitable for use as an electrical contact material wherein the ratio of refractory material to soft metal material is substantially uniform throughout the composite material.

Yet another object of the present invention is to provide a reinforced metallic composite material suitable for use as an electrical contact material wherein the refractory material used to reinforce composite material is about 55 to about 90% by volume of the total volume of the reinforced metallic composite material.

Further'objects will become apparent upon further review of the appended specification. Other objects will appear in the following description, appended claims and appended drawings.

In the drawings:

FIG. 1 is an enlarged cross sectional view of a composite of a laminate structure suitable for use as a contact material; g I

FIG. 2 is an enlarged partial cross-sectional view of a composite material having a refractory metal powder skelton wherein the refractory metal'is substantially surrounded with a soft metal material;

FIG. 3 is an enlarged partial cross-sectional view of the contact material of the present invention illustrating substantially continuous refractory metal wires substantially perpendicular to the working face of the contact material; I

FIG. 4 is a top view of the contact material illustrated in FIG. 3; and I FIG. 5 is a partial sectional view of the contact material shown in FIG. 3 illustrating an array of'the refractory wire material wherein the voids between adjacent wire materials are substantially filled with asoft, highly conductive metal.

. Generally speaking, the present invention relates to a reinforced composite material suitable for useas an electrical contact material. Refractory metal wires having a substantially continuous length and a diameter of FIG. 1 illustrates a laminated composite material 10 consisting essentially of a body 11 of a refractory material ;such as tungsten which is mechanically bonded by any suitable means such as by pressure welding to a soft material l2 such as copper. The working surface of the contact material is composed substantially entirely of soft metal. The entire outer periphery of the body 11 may be coated with copper. Although the contact illustrated in FIG. 1 is stronger than a contact fabricated entirely from a soft metal, the contact material of FIG. 1 may experience harmful erosion'due to arcing since the working'surface is a soft metal.

FIG. 2 illustrates a composite material 20 wherein a refractory metal particle 21 such as tungsten of a tungsten skeleton is substantially surrounded by a matrix of soft metal 22 such as copper. The structure illustrated in FIG. 2 affords a working surface which is composed of both a refractory metal and a soft metal thereby resulting in a compromise between the strength of the refractory metal and the conductivity of the soft metal. However, in order to maximize the compromise between the properties of composite contact material, it is essential that the working surface have a ratio'of refractory metal to soft metal that falls within a desired range. For example, if the percent by volume of the soft metal constituent exceeds the desired maximum, the soft metal may have a tendency to erode rapidly from the working surface of the composite material due to arcing thereby resulting in excessive pitting during the operation of the contact material. If the percent by volume of thesoft metal constituent falls below the compromise value, the effective resistivity of the composite material exceeds the desirable value.

FIG. 3 illustrates the reinforced metallic composite material 30 of the present invention. Refractory metal fibers or wires 31 are aligned so that the longitudinal axis of each of the refractory metal wires may be substantially parallel with the longitudinal axis of the contact material. If working surface 32 is perpendicular to the longitudinal axis of the contact material, the metal wires will be substantially perpendicular to the working surface 32 of the contact material. The working surface may be formed at an angle other than perpendicular to the axis of the refractory metal wires. Re-

fractory metal wires perpendicular to the working surface are preferred. It should be noted that the refractory metal wires are substantially continuous. Thereabout Ito about 50 mils are used in the composite'material. The wires may be touching or may be separated by up to about 40 mils. The voids between said wires are substantially filled by a soft metal which has high electrical and thermal conductivity and which is substantially mutually insoluble and substantially nonreactive with the refractory metal. The composite material may be prepared by reducing atmosphere or vacuum infiltration techniques or by coating the refractory metal wires with the soft metal and heating an array of coated refractory wires to above or below the melting point temperature of the soft metal to provide a bonded mass of refractory wires substantially surrounded by a matrix of soft metal.

fractory metal wires may be tungsten, molybdenum, niobium, rhenium and tantalum. Of the several refractory metals, tungsten and molybdenum are preferred with tungsten being the most preferred refractory metal. The void spaces between the adjacent refractory wires are substantially filled by a soft metal 33 having the properties of high electrical and thermal conductivity and being substantially mutually insoluble and substantially non-reactive with the refractory metal such as copper, silver, gold, zinc, alloys thereof and the like. Of the several soft metals copper and silver are preferred with copper being the most preferred metal.

The contact made of material containing the aligned refractory metal wires may have an axial length and a diameter reduced by about 20% over that of the composite material illustrated in FIG. 2 to achieve about the same contact behavior whenboth contact materials are to be subjected to substantially the same operating conditions. The reduction in thickness and diameter of the contact for the aligned wire material is the result of,

among other things, the reduced erosion rate enjoyed by the contact material of the instant invention. The diameter of the refractory metal wires is about 1 to about 50 mils, a diameter range of about 5 to about mils is preferred.

The diameter of the refractory wire is important in determining the strength to volume ratio (smaller diameter wires are generally stronger than larger diameter wires) and in establishing the dimensions of the surface areas of the two. phases of the composite. The contact stresses which appear during the electrical switching operations are generated by the high pressures and high temperatures on the surface and tend to be' a maximum in a direction perpendicular to the working surface of the contact material. Aligned small diameter wire composites have maximum strength in the same direction.

The desired contact behavior of a composite material is approached if the soft metal phase and the refractory metal phase contains no point in which one phase is much further away from the other phase than the radius of the cathode arc spot which may be generated during the electrical switching operation. The size of the arc spots appear to be dependent on the materials involved and on the electrical power and current levels present during the switching operation. Arc spots also exhibit high mobility, that is, movement about the working surface of the contact. For a soft metal such as copper, are current densities from about 3000 amps/cm to about 120,000 amps/cm at current levels of about 5 to about 20 amperes and ofabout l0 amps/cm at a current level of about 1400 amperes have been observed. Other soft metals show similar electrical properties. The current densities for refractory metals such as tungsten vary from about 14,000 amps/cm to about 90,000 amps/cm at current densities of less than about amperes. The refractory metal and soft metal composite contact material may be expected toexhibit an arc spot diameter of about 5 to about 50 mils. Therefore, to satisfy the criterion that the soft-metal phase and therefractory metal phase contain no points much further away from the other phase than the radius of the are spot, it is important that the diameter of the refractory wire be about 1 'to about 50 mils and separation between wires be limited to about three-fourths of the diameter of the wire.

The maximum percent by volume of refractory wire is about 90% and is achieved when the refractory metal wire is in a hexagonalclose packedarray. The minimum volume percent of refractory metal wire to satisfy the minimum phase dimensional requirement would be an expanded hexagonal array with a maximum separation of the wires ofaboutthree-fourths the diameter of the wire. This minimum volume of refractory metal is about 30%. In general, refractory metal, soft metal composites having more than 90% by volume or less than 30% by volume of refractory metal, do not show the desirable combined properties of the two components. The useable composition range of the reinforced metallic composite is therefore about 90% to about 30% by volume of refractory metal wire, the remainder being essentially soft metal. The preferred range of refractory metal wire in the composite is about. to about by volume.

The reinforced electrical contact material may be prepared by infiltrating. the soft metal into a sintered or unsintered refractory array of wires or fibers by heating the refractory array and the soft metal to a temperature at or several degrees above the melting point temperature of the soft metal in a-reducing atmosphere such as hydrogen or dissociated ammonia. The soft metal infiltrates the void spaces between the refractory wires or fibers. For example, an array of tungsten wires may be contacted with copper and placed in a hydrogen atmosphere and heated to about 1100C to about l250C. The copper substantially completely infiltrates the tungsten array. The soft metals such as copper, silver, gold, zinc or alloys thereof may be used to infiltrate any one of the refractory arrays of tungsten, molybdenum, rhenium, niobium or tantalum. The temperature used to infiltrate the soft metal into the array is governed by the metal point temperature of the metal or of the metal alloy.

if a reinforced contact material of low gaseous content and component content which may release gas is desired, vacuum infiltration techniques may be employed. The 'contentof gaseous component and of components which may release gas in a component prepared by vacuum infiltration techniques is generally less than 10 ppm (parts per million)'and usually less than 2 ppm. Using vacuum infiltration techniques the contact material may be prepared by bundling the refractory metal wires of the chosen diameter, between 1 and 50 mils, into a closed packed array. The bundle of wires is placed in a vacuum environment having a pressure of 10 torr or harder, preferably 5 X 10*.torr or harder. The wires are contacted with a molten soft metal heated to a temperature above the melting point temperature of the soft metal, which for copper would be a temperature of about l250 for a time of about 30 minutes to about 1 hour. If the melt is at a temperature of less than about C above the melting point temperature of the soft metal, (1083C m.p. for copper), poor fluidity of the melt will be experienced and themfiltration time lengthens considerably. I

The refractory metal wire may be pretreated. in a reducing atmosphere such as hydrogen, dissociated ammonia or thelike which is heated to a temperature below the sintering temperature of the refractory metal but at least about 800C to about 1000C for about one-half hour or longer. Useful pretreatmentatmospheres-for this processincludes those atmospheres which do not react with the refractory metal to produce deleterious or. obnoxious products. Pretreating the refractory metal wire removes deleterious surface products thereby assisting infiltration of the refractory metal wire bundleby'the soft metal. Pretreating the refractory metal componentalso appears to assistin lowering to less than 10 ppm the content of gaseous components and components which may be converted to gas. Vacuum degassing the pretreated refractory metal array at a temperature about 200C higher than the bility of recontamination. However, the wire may be pretreated prior to forming the array if desired.

An alternative process for manufacturing the composite material is (a) to coat the refractory metal wire with the soft metal, (b) to bundle the coated refractory wire into an array, and (c) to heat this bundle to a temperature above the melting point of the soft metal resulting in a unitary composite material. To preserve the initial geometric array of refractory wires in this process a zone melting technique may be used.

Another process for making the composite material is to wind the wire about a removable mandrel with the wound wire being in a close packed array. The wound wire bundle is then impregnated with the molten soft metal. Contact material may then be obtained from the finished impregnated solid.

When the vacuum infiltration technique is used to prepare the composite, a small but significant amount of a reactive metal such as titanium or zirconium may be used to assist infiltration of the soft metal into the refractory metal array. Of the several reactive metals, titanium is the preferred reactive metal."The reactive metal appears'to afford wetting of the refractory metal in the instance where the refractory metal does not undergo the pretreatment step. The reactive metal also provides a getter function by removing material from the gaseous phase within-the vacuum device during subsequent operation as a vacuum contact; The reactive ,metal content of the reinforced metallic material is from 0.0l% to about-0.4% by volume and-this comper. Lengths of about mil tungsten wire are cleaned with a suitable solvent such as sodium peroxide. The wires are bundled in such a way so as to provide an array such as'that shown in FIG. 5. The wires are contacted with copper in a vacuum environment having a pressure of 5 X 10 torr or harder. The tungsten wire array is vacuum infiltrated by heating the tungsten and the copper to a temperature that exceeds the melting point temperature of copper, that is, about 1250C to thereby cause infiltration of the copper melt into the tungsten compact. For test contacts the tungstencopper composite material is divided into lengths of about three-eighths' of an inch long having the longitudinal axis of the tungsten wire perpendicular to the working surface of the composite material such as that shown in FIG. 3. This composite material tended to exhibit lower erosion rates than contacts of equal composition made ponent may be conveniently added to the system by alloying with the soft metal. Exceeding about 0.4% by volume of reactive metal may result in the soft metal having an undesirably high resistivity and falling below about 0.01% by volume does not seem to assist infiltration of the soft metal into the refractory metal array or in the getting action of gases. The percent by volume of the constituents of a reinforced metallic material containing a reactive metal would be about 30% to about 90% refractory metal, about 69.99% to about 9.99% soft metal and about 0.01% to about 0.04% reactive metal.

As an alternate method of introducing the reactive metal into the system the refractory metal wires may be coated with the reactive metal by electrolytic or vapor phase plating prior to contacting. the refractory metal array with the molten soft metal.

Additions of other metals to the .soft metal melt or to the refractory array by coating the individual wires prior to infiltration, whether using-a hydrogen atmosphere or a vacuum environment, may be made to improve the electrical contact characteristics of the composite material. For example, small but significant amounts of metals may be added-to the melt which enhance some of the desirable properties of the material such as the vapor pressure, corrosion resistance, machinability and the like. Ari example of such metals are bismuth and tin which enhance the vapor pressure propertyof the contact material.

it is believed that the invention will be better understoodfrom the following examples whichare illustrative of the present invention and not limiting thereof.

EXAMPLE 1 with tungsten powder.

EXAMPLE 2 A composite material consisting essentially of about 78.5% by volume tungsten, the remainder copper. Lengths of about 5 mil tungsten wire are cleaned with sodium peroxide. The wires are bundled in such a manner so as to provide a compact array. The array'is pretreated in a hydrogen atmosphere at about 1000C for about 1 hour. The array is degassed at about 1200C until a'pressure of 5 X 1 0' torr is realized. The array is infiltrated with copper at about l250C in the vacuum environment. i i 1 EXAMPLE 3 A composite material essentially of about by volume tungsten the remainder essentially copper. Lengths of about 5 mil tungsten wire are cleaned with a suitable solvent such as sodium peroxide. The wires are bundled in such a way so as to provide an array such as that shown in FIG. 5. The wires are contacted with copper in a hydrogen atmosphere. The tungsten wire array is infiltrated by heating the-tungsten and the copper to a temperature that exceeds the melting 'point temperature of copper, that is about 1250C to thereby cause infiltration of the copper melt into the tungsten compact. For test contacts thetungsten-copper composite material is divided into lengths of about three- While the invention is illustrated and described in embodiment, it will be understood that modifications and variations may be effected without departing from the scope of the concepts involved in this invention and set forth in' the appended claims. I

Having thus described our invention, we claim:

1. A method of making electrical contact material having a low gaseous component content and components which may beconverted to gas which consists essentially of 30 to volume percent refractory metal, the remainder of soft metal wherein the refractory metal is an array of refractory metal wires having a diameter of l to 50 mils substantially perpendicular to the working surface 'of the contact material, the method comprising the steps of providing a closely packed bundle of refractory metal wires pretreated in fect at least some cleansing thereof, vacuum degassing the bundle of refractory metal wires at a temperature above the pretreatment temperature but below the sintering temperature of the refractory metal to reduce the content of gaseous components and components which may be converted to gas to less than lppm, and contacting the bundle of refractory metal wires with molten soft metal in a vacuum to infiltrate void spaces between the refractory metal wires to provide contact material having the adjacent refractory metal wires separated by no more than 0.75 of the diameter of the largest refractory metal wire at the working surface of the electrical contact material.

2. A method according to claim 1 in which said pretreating is carried outat temperatures of about 800 to 1000C.

3. A method according to claim 1 wherein said reducing atmosphere is selected from hydrogen, dissocitungsten, molybdenum, rhenium, niobium and tantalum.

7. A method according to claim 6 wherein said soft metal is selected from the group consisting of copper,

silver, gold, zinc and alloys thereof.

UIHTEDSTATES PATENT OFFICE v v CERTIFICATE OFjCO-RRECTION PATEM'? NO. r 3, 52,379 r DATED 1 12/10/74 INYVENTOMS) 1 v Richard H. Krock and Edward J. Zdanuk it is certified that error appears in the above-identifiedpatent an'd that said Letters Patent are hereby corrected as shown below:

Col. 8, line 11 Insert a hyphen between "tungsten" and II Col. 8, line 33 Insert --c onsistingafter "material" Signed and sealed this 20th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks 

2. A method according to claim 1 in which said pre-treating is carried out at temperatures of about 800 to 1000*C.
 3. A method according to claim 1 wherein said reducing atmosphere is selected from hydrogen, dissociated ammonia, and mixtures thereof.
 4. A method according to claim 1 wherein said vacuum degassing treatment is continued until a pressure of 5 X 10 5 torr or lower is obtained.
 5. A method according to claim 1 in which said vacuum treatment is carried out at a temperature of about 200*C above said pre-treating temperature but below the sintering temperature of said refractory metal.
 6. A method according to claim 1 wherein said refractory metal is selected from the group consisting of tungsten, molybdenum, rhenium, niobium and tantalum.
 7. A method according to claim 6 wherein said soft metal is selected from the group consisting of copper, silver, gold, zinc and alloys thereof. 